chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/l10n.yaml

@@ -0,0 +1,11 @@
+# Flutter's gen-l10n: the strings in lib/l10n/*.arb become the AppLocalizations class,
+# generated into lib/l10n/app_localizations*.dart (committed, never edited or formatted
+# by hand). app_en.arb, the template, is British English, also the fallback;
+# app_en_US.arb overrides what American English spells differently; app_en_GB.arb is
+# empty, there so that Material and dates format the British way; app_de.arb is German.
+arb-dir: lib/l10n
+template-arb-file: app_en.arb
+output-localization-file: app_localizations.dart
+output-class: AppLocalizations
+# `AppLocalizations.of(context).foo` without a `!`: the delegate is always installed.
+nullable-getter: false